256 



THALLOPHYTES. 



resemblance in their mode of origin and subsequent development to those of the 

 Pandorineae. When the ccenobia multiply, a large number of zoogonidia are formed 

 in each mother-cell, within which they move about for some time ; when they come 

 to rest they congregate in some definite arrangement, and then continue their develop- 

 ment unitedly, eventually again multiplying in the same way, as for instance in Pediastrum 

 (Fig. 169). In Hydrodictyon utriculatum, which occurs occasionally in ditches, the mature 



Fig. Tb^.— Pediastrum gratiidatttm (after A. Braun, x 400). A a plate consisting of 

 united cells ; at ^ the innermost layer of a cell-wall is protriKling- ; it encloses tlie 

 daughter-cells resulting from division of the green protoplasm ; at t are varions states of 

 division of the cells; sp the fissures in the already empty cell-walls ; B the inner lamella 

 of the mother-cell-wall which has entirely escaped (greatly enlarged) ; b contains the 

 zoogonidia g, these are in active ' creeping ' motion ; C the same family of cells 

 4^ hours after its birth, 4 hours after the zoogonidia have come to rest ; these have 

 arranged themselves into a plate, which is already beginning to develope into one 

 similar to A. 



plant or coenobium consists of a sac-like net several centimetres long, which is composed 

 of a great number of cylindrical cells united at their ends so as to form a four- or six- 

 cornered mesh. The ordinary mode of reproduction consists in the green contents 

 of one of the cells of the net breaking up into from 7,000 to 20,000 zoogonidia which 

 move about with a trembling motion within the wall of the mother-cell, come to rest in 

 the course of half-an-hour, and then arrange themselves in such a way that, by their 

 elongation, they again form a net of the original kind which is set free by the absorption 

 of the wall of the mother-cell, and attains, in the course of three or four weeks, the size 

 of the mother-plant. In other cells of the mature net the green contents break up 

 into from 30,000 to 100,000 microzoogonidia which at once leave the mother-cell and 

 swarm about for some hours. The hypothesis has not yet however been confirmed 

 that conjugation then takes place between these zoogonidia^ ; but when they come to 

 rest they are spherical, invested with a firm cell-wall, and may retain their vitality for 

 months when dried up if protected from light. After remaining several months at rest 

 these resting-spores begin to grow slowly, and after they have attained a considerable 

 size their contents break up into two or four large zoospores which come to rest 

 after a few minutes, and assume a peculiar angular form when they have reached a 

 considerable size, putting out horn-like appendages. In each of these so-called poly- 

 hedra the green parietal protoplasm again breaks up into zoogonidia which move about 

 for 20 or 40 minutes within a sac which protrudes out of the polyhedron. When come 

 to rest they arrange themselves into a sac-like net consisting of from 200 to 300 

 cells, but in other respects resembling one of the ordinary ones. In some of the 

 polyhedra smaller and more numerous zoogonidia are formed, but these also unite 

 into a net. 



[See note supra, y>. 251,] 



